The invention relates to a process for the preparation of hydrocarbons by catalytic reaction of carbon monoxide with hydrogen.
The preparation of hydrocarbons from a H.sub.2 /CO mixture by contacting this mixture at elevated temperature and pressure with a catalyst is known in the literature as the Fischer-Tropsch hydrocarbon synthesis. Catalysts suitable for the purpose are those comprising cobalt supported on a carrier. Such catalysts can be prepared by keeping particles of a porous carrier in contact with a solution of a cobalt compound for a considerable period of time, then removing the solvent and calcining and activating the composition obtained. Thus are generally obtained catalyst particles in which the cobalt is homogeneously distributed over the carrier material, viz. at every point of the catalyst particle the cobalt concentration present is virtually the same.
An investigation into the use of catalysts comprising cobalt on a carrier for preparing hydrocarbons from H.sub.2 /CO mixtures has found that the C.sub.5.sup.+ selectivity (selectivity to obtain C.sub.5 and higher boiling hydrocarbons in the product) of these catalysts is highly dependent on the way in which the cobalt is distributed over the carrier material. It has been found that catalysts in which the cobalt is inhomogeneously distributed over the carrier material show a much higher C.sub.5.sup.+ selectivity than similar catalysts where the cobalt is distributed homogeneously over the carrier, provided that said inhomogeneous distribution meets certain requirements. In order to assess the inhomogeneity of the cobalt distribution over the catalyst particles, the latter are taken to be composed of a kernel surrounded by a peel, the kernel being of such a shape that at every point of the kernel perimeter the shortest distance (d) to the perimeter of the peel is the same and that d is equal for all catalyst particles under consideration and has been chosen such that the quantity of cobalt present in the total peel volume (.SIGMA.V.sub.p) is 90% of the quantity of cobalt present in the total volume of the catalyst particles under consideration (.SIGMA.V.sub.c). It has been found that the catalysts having an inhomogeneous cobalt distribution show no significantly improved C.sub.5.sup.+ selectivity as related to catalysts with homogeneous cobalt distribution, unless the inhomogeneous cobalt distribution is such as to meet the requirement EQU (.SIGMA.V.sub.p /.SIGMA.V.sub.c)&lt;0.85
For determining .SIGMA.V.sub.p the "electron microprobe analysis" method can very suitably be used. The electron microprobe analysis procedure is described in Instrumental Methods Analysis D. Van Nostrand Co., New York, (1974) 5th Ed. H. H. Willard et al, Ed. page 272.